Add file checks to digfort

Checking file type and existence before we start avoids a few issues,
and prevents (highly) misleading feedback of "done" regardless of action
taken.  Closes #453

NEWS

@@ -1,6 +1,7 @@
 DFHack Future
     Internals
     Fixes
+        digfort.rb: check file type and existence before starting
     New Plugins
     New Scripts
     Misc Improvements

scripts/digfort.rb

@@ -1,7 +1,22 @@
 # designate an area for digging according to a plan in csv format
 
-raise "usage: digfort <plan filename>" if not $script_args[0]
-planfile = File.read($script_args[0])
+fname = $script_args[0].to_s
+print(fname)
+
+if not $script_args[0] then 
+    puts "  Usage: digfort <plan filename>"
+    throw :script_finished
+end
+if not fname[-4..-1] == ".csv" then
+    puts "  The plan file must be in .csv format."
+    throw :script_finished
+end
+if not File.file?(fname) then
+    puts "  The specified file does not exist."
+    throw :script_finished
+end
+
+planfile = File.read(fname)
 
 if df.cursor.x == -30000
     puts "place the game cursor to the top-left corner of the design"